Sweet - I always wanted to play these (never had a Mac). Are there any Windows source ports or worthy Mac emulators that would let me play them? Looks like they're planning to release Windows versions eventually.

Aleph One is an open source reimplementation of the Marathon 2 engine (released some moons ago by Bungie). It can play M2 and M3 resources natively. M1 resources have to be converted beforehand, or you could just download M1A1. As a neat bonus to the trilogy, I highly recommend the unofficial Marathon 4 (more than five years in the making).